This invention relates to facsimile transmission over packet networks. In particular, the present invention relates to the repair of scan line errors in fax over packet transmissions.
When dedicated facsimile devices communicate directly, a series of command signals are exchanged which allow the devices to communicate. The control information includes training to accommodate the speed and specifications available at both ends of the transmission and communications to ensure that the connection is not corrupted or interrupted. The exchange of data is direct, without delay or translation. Because of this direct connection, facsimile equipment is designed to function properly with a constant connection and not in an environment where the connection is intermittent or interrupted. If a facsimile device detects an interruption/error in transmission, the facsimile device interprets this as a transmission error and will time out and disconnect or attempt to retrain the current transmission. Retraining, if successful, will often result in a slower transmission speed in an attempt to avoid additional errors, hang-up and/or loss of transmission. Newer facsimile devices may also incorporate error correction mode capabilities which will compensate for limited interruption/error in transmission of image data.
When facsimile devices are connected through a packet network, they are not directly connected. The facsimile information is packetized at a sending end, sent over the packet network and reassembled into a facsimile format at the receiving end before presentation to the receiving facsimile device. The Connection over a packet network is not a constant connection and often experiences line delays, errors and/or packet loss which must be hidden from the facsimile device by a gateway in order to avoid error detection by a facsimile device attempting to communicate over a packet network.
Typically, low speed (T.30 hand shaking) control data is protected by simple redundancy techniques in fax over packet applications to combat network packet loss. The repetition of control data helps to insure that the facsimile devices receive an essentially uninterrupted control signal even in the event of packet loss, essentially fooling the facsimile device into believing that it has an uninterrupted direct constant connection to the facsimile device on the other side of the connection. The repetition of control data has negligible network bandwidth impacts because of the small size of control data and produces significant benefits to call success rates.
High-speed, large size, image data is not protected through repetition, because of undesirable increases in network bandwidth requirements. Prior to the present invention, the loss of image data causes page errors in the receiving facsimile device, causes loss of connection and excessive retraining which can significantly increase the time of transmission and loss of image data results in errors in the printed document. Error Correction Mode (ECM) employed by most newer fax devices can combat packet loss to a limited extent but may only result in the problems associated with excessive retraining and retransmissions.
In ECM mode of operation, when network packet loss exceeds two to three percent, ECM becomes ineffective and leads to consistent call failures when high-speed data is not protected over the network. When packet loss exceeds two to three percent, it has been observed that there is continuous data retransmission requests and re-trains. Eventually, the facsimile device gives up and terminates the call unsuccessfully. Disabling the ECM mode in packet loss environments exceeding two to three percent would improve performance.
In non-ECM modes of operation, the high-speed image packet loss translates to image errors at the receiving facsimile device. Newer facsimile devices will employ techniques to compensate for these page errors, but retrains can occur due to perceived bad line qualities. The retrains lead to longer call durations and as the number of pages to send via facsimile increases, so does the likelihood of a call failure due to excessive retrains. Also, in the field, poor legacy facsimile devices have been observed which cannot tolerate any received page errors. These facsimile devices terminate the call abruptly and go on hook in the middle of a page if packet loss occurs.